The present invention relates to a method and a control unit for setting an internal combustion engine turbocharger turbine flow cross-section. DE 198 21 902 A1 describes a boost pressure regulator for an exhaust gas turbocharger having variable turbine geometry (VTG) in an internal combustion engine. Diesel engines and split injection and direct injection gasoline engines are cited as examples of internal combustion engines. Furthermore, the boost pressure regulator, which is typically implemented as a PI regulator, must be limited in order to avoid continuous integration of the regulator when stationary precision is not achieved. The adjustment range of the regulator is greater than the setting bandwidth of the boost pressure, so that the boost pressure no longer reacts to manipulated variable changes at the boundaries of the adjustment range of the regulator. Dynamic limiters controlled by ignition maps are known as a remedy, but these are complex and do not always result in optimum results. Input variables of such a limiter controlled by ignition maps are not specified. An undesired boost pressure overshoot in the event of a load jump is particularly disadvantageous.
In order to avoid such boost pressure overshoots in non-stationary operating states without impairing the regulating behavior in stationary operating states, said DE 198 21 902 also suggests that the regulating range be limited in non-stationary operating states as a function of a filtered reference variable of the boost pressure regulator, preferably one which characterizes the load. An injected fuel quantity of a diesel engine is cited as an example of such a reference variable.
Avoiding boost pressure overshooting in the event of a sudden rise of a load of the internal combustion engine represents one optimization criterion among other possible optimization criteria. Under specific conditions, it may be just as important or even more important to allow the most rapid possible torque build-up in the event of a positive load change. A positive load change is understood as any load change having an increase of the load, with the load largely correlating with the torque generated by the internal combustion engine.
Reducing overshooting by limiting the regulator manipulated variable may also result in the undesired increase in the delay with which the actual value of the torque follows a change of its setpoint value in a non-stationary operating state. Such a delay in an internal combustion engine of a motor vehicle is disturbing to the drivability and the subjective driver impression which the motor vehicle provides.
An object of the present invention is to provide a method and a control unit, by which the most rapid possible torque build-up may be achieved in a non-stationary operating state having increased torque demand.
This object has been achieved in a method and a control unit by using the speed of the internal combustion engine as the operating parameter as a function of which a range of settable values of a turbine flow cross-section are determined.
In principle, it is surprising that the demand for the most rapid possible boost pressure build-up is at all compatible with limiting the turbine flow cross-section at the bottom, i.e., with determining a lower limit of a range of settable values of the turbine flow cross-section as a function of an operating parameter. Rather, one would ordinarily expect that the boost pressure would build up most rapidly precisely when the largest possible part of the exhaust gas energy is transmitted to a turbine wheel of the turbocharger, which is more the case at a smaller turbine flow cross-section than at a larger turbine flow cross-section.
At predefined exhaust gas mass flow, the value of the transmitted energy increases with rising pressure gradient over the turbine. Because the pressure of the exhaust gases is essentially constant after flowing through the turbine and is only slightly higher than the ambient pressure, the pressure gradient is a function above all of the damming effect of the turbine, which effect is maximal at minimal turbine flow cross-section.
The exhaust gas counterpressure which results in the combustion chambers of the internal combustion engine with opened exhaust valves, however, also rises with the damming effect. Because inlet valve and exhaust valve of a combustion chamber are opened jointly during the valve overlap, the increased exhaust gas counterpressure obstructs the inflow of air or fuel/air mixture with open inlet valve. Therefore, the charge of the combustion chambers with combustible mixture and thus the torque development are unfavorably influenced by a high exhaust gas counterpressure.
In addition, the exhaust gas quantity and heat energy flowing to the turbine drops when the quantity of the exhaust gas returned into the combustion chambers rises. Even if a maximum energy component is transmitted to the turbine wheel, viewed relatively, the absolute value of the transmitted energy is not optimum under certain circumstances, resulting in a delayed build-up of the charge pressure.
The inventors have recognized that this delay is not solely a function of the value of the exhaust gas counterpressure, but rather the delay is correlated more with another engine parameter. This engine parameter, in which optimum and suboptimum torque and boost pressure curves differ, is the scavenging gradient. This term refers to the difference p2−p3 of the pressure p2 in the direction of the gas flow before the inlet valve and the pressure p3 after the exhaust valve.
The scavenging gradient is negative in the most unfavorable case. In other words; the pressure p3 is greater than the pressure p2. In principle, the scavenging gradient may be measured or modeled from a larger or smaller number of operating parameters of the internal combustion engine. It has been shown, however, that the value of the scavenging gradient at which an optimally rapid boost pressure and torque increase results varies only slightly and may be described as a function of a few parameters.
The scavenging gradient is itself strongly dependent on the turbine flow cross-section in the exhaust gas mass flow, however, and this is in turn correlated with the speed. Viewed qualitatively, the scavenging gradient rises with increasing speed and falls with growing turbine flow cross-section. Therefore, for a specific speed, there are turbine flow cross-sections at which a critical scavenging gradient does not yet occur, and other, smaller flow cross-sections at which a critical scavenging gradient already occurs.
Speed is an operating parameter which is detected and analyzed in any case in modern internal combustion engines. The use of speed as an operating parameter for determining the lower limit of a range of settable values of the turbine flow cross-section therefore allows optimum setting of the turbine flow cross-section in regard to the most rapid possible boost pressure and torque increase in a non-stationary operating state without additional sensors. This is true independently of whether the present invention is viewed in its method aspects or in an implementation in the form of a control unit set up by appropriate programming.